1. Field of the Invention
The invention relates to an extrusion die assembly, more particularly to an extrusion die assembly for extrusion of a plate having a multi-layered structure or a core-and-shell structure.
2. Description of the Related Art
It is known to produce extruded plates having a multi-layered stack structure or a multi-layered core-and-shell structure. The multi-layered stack structure has differently colored layers disposed one on top of the other. The multi-layered stack structure is simple, and can be extruded easily using the known extrusion method. On the other hand, the multi-layered core-and-shell structure has one or more shell layers covering an outer surface of a core. Since it is very difficult to control the material of the shell layer to uniformly cover the outer surface of the core, the current method includes extruding a core first, after which the core is passed through a second extrusion die to form the shell layer. When a core-and-shell structure having four shell layers is to be manufactured, four extrusion steps must be carried out. Hence, such a method for extruding multi-layered core-and-shell structures is time-consuming, and the processing steps are complicated.